If it had been porosity I would have twigged straight away, but it was just feeding problems. We tried and blamed a few different machines and setups, but it came back to the wire in the end, after we sorted a bulletproof machine setup. Could possibly have blamed the wire itself I suppose, but dimensionally it was all good stuff... Went from old Murex 1050 to new packs of Esab 1080 and the burnback problems went from once every 300mm of weld, to practically non existant.
This is always my rescue plan if I run out of mig mix. Flip a CO2 extinguisher upside down and away I go welding again.
Same goes for regular CO2 bottles, dip tube or not makes the difference. If the regulator sucks liquid CO2 it will burst. So flip it upside down to take only gas for CO2. The bottles usually have marking stating the siphon or not.
